U.S. Pat. No. 2,947,639 pertains to an immersion tin plating coating composition which utilizes metal phosphates to reduce the formation of tin dioxide sludge. The '639 patent utilizes compositions that are highly undesirable from an environmental perspective. Phosphate systems are to be avoided due to substantial environmental concerns.
Previous tin coating compositions had a tendency to produce insoluble sludge or particles which were highly undesirable. Such coating compositions resulted in inconsistent coating films as well as poor adhesion of the tin film onto the substrate, in particular, an aluminum substrate.
When shaped articles of aluminum such as pistons and the like were coated with tin, frequently poor adhesion of the coating was the result due to the sludge that is present in the bath.
Other difficulties associated with coatings for aluminum would be the use of environmentally undesirable materials such as chromates or cyanide and the like. In addition, coatings have been applied to metallic substrates such as aluminum or other light metals as magnesium and titanium by an electrolytic technique. Such electrolytic techniques have substantially different problems associated with it not the least of which is having the parts properly handled from an electrolytic perspective, grounding of the tank and the overall difficulties associated with an electrolytic system, such as safety hazards with such current flows.
It has been described previously that one of the most significant problems encountered in attempting to tin plate aluminum castings resides in the aggressive nature of hot alkaline (pH=13), tin plating solutions wherein the solution tends to attack the porous cast aluminum substrate producing excessive accumulations from corrosion products and severe etching of a cast aluminum substrate; both constituting problems which result in severe blistering, peeling and/or flaking of the plated deposits (U.S. Pat. No. 4,346,128, columns 18-19).
Most tin coating compositions have generally been on the acidic side due to the perceived difficulties with alkaline compositions (see U.S. Pat. Nos. 4,550,037, 4,935,312, and 4,715,894).
It is an object of the present invention to provide a good tin coating onto metallic substrates such as aluminum and aluminum alloys with good adhesion and a decrease in sludge formation at alkaline pH.
It is an object of the present invention to apply alkaline tin electroless coatings to aluminum and aluminum alloys utilizing chelating agents that are comprised preferably of polyamine materials, such as amino carboxylic acid materials.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention.